BAFF (B cell activating factor, BLyS, TNFSF13B) is a TNF-family cytokine produced by a number of cell types such as dendritic cells (DCs), monocytes, neutrophils, T and B cells and stromal cells. BAFF is essential for mature B cells to develop and to respond to antigens (Ags). Dysregulation of BAFF expression has been implicated in the development of a number of immunologic diseases, most notably autoimmune diseases like systemic lupus erythematosus (SLE). New drugs are being developed and tested for treating diseases associated with BAFF dysregulation. However, relatively little is known about when, where and how BAFF is produced in vivo and about which BAFF-producing cells contribute to B cell responses and B cell associated diseases. We will develop two tools for monitoring tissue and cell specific expression of BAFF and for assessing which BAFF-producing cells contribute to immune responses and immunopathology. We have two Aims: Aim 1. Generation of BAFF-RFP reporter and BAFF/BLyS knockin (KI) mice. Using a dual recombinase vector we will first generate a transgenic (Tg) line expressing a bright, highly stable monomeric form of red fluorescent protein (RFP) designated TagRFP-T on one allele of the Baff (Tnfsf13b) locus. We will also create a BAFF KI line (BAFFfl/+ mice) and cross these mice with appropriate B6-Cre lines to create knockout (KO) mice lacking BAFF expression in either DCs or B cells. Aim 2. Determining the role of BAFF derived from DCs or B cells in B cell development and humoral immune responses. We will compare the antibody responses of WT mice, BAFF DC KO mice and BAFF B cell KO mice immunized with either a T cell-dependent Ag or a T cell-independent Ag. We will also monitor the expression of BAFF in different cell types over time after immunization using BAFF-RFP reporter mice. The proposed studies are highly significant and have the potential to have high impact since the BAFF-RFP reporter and BAFFfl/fl KI mice will greatly enhance basic B cell biology research and enable pre-clinical studies to be carried out to identify functionally important sources of BAFF and the effect of therapies on BAFF production and function.